[Feature Request] Add `customIconPath` configuration for custom icons

[Antyos]

This is a continuation of the discussion following the closing of #984.

Instead of specifying a relative path from the icon directory of this extension, it would be useful to have a separate configuration to set a path (preferably absolute, but relative works too) to a folder containing custom icons. Then, each icon could be specified just by its name instead of an entire path.

The proposed config might look like:

"material-icon-theme.customIconPath": "~/.vscode/icons",
"material-icon-theme.files.association": {
    "fileName.ts": "sample"
}

For the icon:

.vscode
 ┣ extensions
 ┗ icons
   ┗ sample.svg

Note: Assuming in this theoretical implementation, the old location for the icons folder works again.

This is heavily dependent on what the VS Code API allows.

Perhaps if the full implementation isn't possible, the extension could at least check a few locations, such as .vscode/extensions/icons/ for icons so the custom icons specified in the configuration don't need the ../../icons/ prefix.